Method for interconnecting close lead center integrated circuit packages to boards

ABSTRACT

The invention is a method for interconnecting close lead center integratedircuit package to boards by forming the leads as follows: (1) bonding the lead frame material to a thin insulating film, and the (2) forming the leads. Next, the film base is removed selectively where connections to the package and board will be made. The lead material is then plated and formed to provide stress relief in the package-to-board interconnect. The lead frame is then attached to the package and board.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuit fabricationtechniques, and more particularly to a method for interconnectingintegrated circuit chip packages having package leads on very closecenters to an interconnect board such that the leads are formed andsoldered on very close centers, yet are protected from handling damageand lead-to-lead shorting.

2. Description of the Prior Art

The trend in integrated circuit manufacture is toward greaterintergration of functions on a single chip. This results in more gateson the chips and higher input/output (I/O) pin counts. I/O requirementsof over 200 leads on a single chip are projected. Packaging of thesechips into a single chip package is desirable due to the high cost ofthe chip, the requirement for testing the chip before committing to anassembly, and the desire to easily remove and replace a defective chipin an assembly. However, current packages require extremely large sizesdue to the package perimeter required for the large number of leads. A200 lead flatpackage with leads on four sides with a standard 50 millead spacing would have a size over 1.5 inches square, resulting in anexpensive, low-density package. Lead centers as small as 12.5 or 10 milson a chip package are desirable. As the lead center decrease, however,the leads become more fragile and susceptible to being bent which causesalignment problems when mounting the package to a board, or evenelectrical shorting if the leads are so badly bent as to touch oneanother. Also, after the package is mounted the fragile leads are stillsusceptible to handling damage and possible shorts.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a method for interconnectingclose lead center integrated circuit packages to boards by forming theleads as follows: (1) bonding the lead frame material to a thininsulating film, and then (2) forming the leads. Next, the film base isremoved selectively where connections to the package and board will bemade. The lead material is then plated and formed to provide stressrelief in the package-to-board interconnect. Finally, the lead frame isattached to the package and board.

Therefore, it is an object of the present invention to provide a methodfor interconnecting close lead center integrated circuit packages tointerconnecting boards which will avoid damage and possible shorting ofleads during handling.

Other objects, advantages and novel features of the present inventionwill be apparent from the following detailed description read inconjunction with the appended claims and attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of a lead frame with close center leadsaccording to the present invention.

FIG. 2 is a perspective view of the lead frame of FIG. 1 after selectivefilm removal.

FIG. 3 is a perspective view of an integrated circuit packageinterconnected to a board by the lead frame of FIG. 2.

FIG. 4 is a top plan view of a lead frame for four-sided leadattachment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The first step in a method for interconnecting close lead centerintegrated circuit packages to an interconnect board is to form a leadframe 10. Lead frame material is sheet form, such as Kovar sheet orcopper sheet, is bonded using standard methods to a thin insulating film12, such as Kapton. The lead frame material and thickness, and the filmbase material and thickness, are so chosen to allow the lead frame 10 tobe later formed in the "Z" axis and maintain its shape due to thedeformation of the metal leads, yet maintain lead-to-lead spacing in the"X" direction due to the film base. The leads 14 are then formed by somemeans such as etching the lead frame material, resulting in the leadframe 10 shown in FIG. 1.

Next, the film base 12 is selectively removed by some means such aslaser ablation to remove material where connections to the package andinterconnect board will later be made. The resulting lead frame 10 isillustrated in FIG. 2. At one end 16 a small strip of material has beenremoved so only a short length of leads 14 are cantilever supported. Analternate method is shown at the other end where a small strip 18 ofmaterial has been left so there is no cantilevered, or unsupported, leadends. The lead material is plated with a suitable material such as goldor solder to prevent corrosion and to aid future soldering operations.

The lead frame 10 is formed in an "S" profile in the "Z" axis as shownin FIG. 3 to provide stress relief in the package-to-board interconnect.A typical interconnect board 20 has a plurality of mounting pads 22. Achip 24 is mounted on a chip package 26, the package in turn beingbonded to the interconnect board 20. A single metalization layer 28 onthe package 26 is etched to form leads from the chip 24 to the edge ofthe package. The lead frame 10 can be either (1) attached to the package26 first, and then package and lead frame attached to the board 20; or(2) the package can be bonded to the board with an adhesive and the leadframe attached to both the package and board. The lead bonding may beaccomplished by soldering, using a gang bonding technique to solder awhole side of leads at once, or even all sides at once. A lead frame 10'for attaching leads to all four sides of a package 26 is shown in FIG.4.

Thus, the present invention provides a method for interconnecting closelead center integrated circuit packages to an interconnect board whichuses an inexpensive, unleaded package with a single metalization layer,and which via a film support minimizes the possibility of lead damagewhile leaving connection joints visible for inspection after soldering.

What is claimed is:
 1. A method for interconnecting a close lead centerintegrated circuit package to an interconnect board comprising the stepsof:forming a lead frame having a plurality of leads bonded to aninsulating film material, said forming step comprising the steps ofbonding a lead frame material to said insulating film material andetching said lead frame material to form said leads, said lead framematerial and said insulating film material being of such materials andthickness so that when said lead frame is subsequently shaped said leadsmaintain their shape while maintaining their lead-to-lead spacing;selectively removing said insulating film material from said leads whereconnections to said package and said board are to be made, saidselectively removing step comprising the step of laser ablating saidinsulating film material to form an exposed portion near the lead ends,retaining a small strip of said insulating film material at said leadends for support; shaping said lead frame to provide stress relief inthe package-to-board interconnect; and attaching said lead frame to saidpackage and board to form the package-to-board interface.
 2. A leadframe for interconnecting a close lead center integrated circuit packageto a interconnect board comprising:an insulating film; a plurality ofleads bonded to said insulating film, said insulating film beingselectively removed from said leads where interconnections to saidpackage and said board are to be made, said leads and said insulatingfilm being of a material and thickness to allow said lead frame to beshaped in the form of an "S" profile in the Z-direction whilemaintaining lead shape and lead-to-lead spacing; said leads comprisingan exposed portion near the ends for attachment to said package and saidboard; and a thin strip of said insulating film being attached acrosssaid ends to support said ends.